Blood enters the heart through two large veins – the posterior (inferior) and the anterior (superior) vena cava – carrying deoxygenated blood from the body into the right atrium.

Blood flows from the right atrium into the right ventricle through the tricuspid valve. When the ventricle is full, the tricuspid valve shuts to prevent blood flowing backwards into the atrium.

Blood leaves the heart through the pulmonic valve into the pulmonary artery and flows to the lungs.

The pulmonary vein carries oxygen-rich blood from the lungs into the left atrium.

Blood flows from the left atrium into the left ventricle through the open mitral valve. When the ventricle is full, the mitral valve shuts to prevent blood from flowing backwards into the atrium.

Blood leaves the heart through the aortic valve into the aorta and to the rest of the body!

As the heart beats, it pumps blood through a system of blood vessels, called the circulatory system. The vessels are elastic, muscular tubes that carry blood to every part of the body.

Blood is essential. In addition to carrying fresh oxygen from the lungs and nutrients to your body's tissues, it also takes the body's waste products, including carbon dioxide, away from the tissues. This is necessary to sustain life and promote the health of all the body's tissues.

There are three main types of blood vessels:

Arteries. Arteries carry oxygen-rich blood away from the heart to all of the body's tissues. They branch several times, becoming smaller and smaller as they carry blood further from the heart and into organs.

Capillaries. These are small, thin blood vessels that connect the arteries and the veins. Their thin walls allow oxygen, nutrients, carbon dioxide, and other waste products to pass to and from cells.

Veins. These are blood vessels that take blood back to the heart; this blood contains less oxygen and is rich in waste products that are to be excreted or removed from the body. Veins become larger as they get closer to the heart. The superior vena cava is the large vein that brings blood from the head and arms to the heart, and the inferior vena cava brings blood from the abdomen and legs into the heart.

This vast system of blood vessels -- arteries, veins, and capillaries -- is over 60,000 miles long. That's long enough to go around the world more than twice!

Blood flows continuously through your body's blood vessels. Your heart is the pump that makes it all possible